全文获取类型
收费全文 | 4978篇 |
免费 | 222篇 |
国内免费 | 31篇 |
专业分类
化学 | 3381篇 |
晶体学 | 53篇 |
力学 | 207篇 |
数学 | 479篇 |
物理学 | 1111篇 |
出版年
2023年 | 18篇 |
2022年 | 90篇 |
2021年 | 105篇 |
2020年 | 82篇 |
2019年 | 95篇 |
2018年 | 59篇 |
2017年 | 66篇 |
2016年 | 155篇 |
2015年 | 144篇 |
2014年 | 201篇 |
2013年 | 338篇 |
2012年 | 366篇 |
2011年 | 393篇 |
2010年 | 267篇 |
2009年 | 215篇 |
2008年 | 335篇 |
2007年 | 267篇 |
2006年 | 286篇 |
2005年 | 246篇 |
2004年 | 205篇 |
2003年 | 175篇 |
2002年 | 198篇 |
2001年 | 105篇 |
2000年 | 99篇 |
1999年 | 63篇 |
1998年 | 51篇 |
1997年 | 36篇 |
1996年 | 60篇 |
1995年 | 49篇 |
1994年 | 46篇 |
1993年 | 46篇 |
1992年 | 33篇 |
1991年 | 43篇 |
1990年 | 27篇 |
1989年 | 29篇 |
1988年 | 25篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 27篇 |
1984年 | 23篇 |
1983年 | 22篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 12篇 |
1976年 | 7篇 |
1974年 | 4篇 |
1972年 | 5篇 |
排序方式: 共有5231条查询结果,搜索用时 9 毫秒
121.
Pyun DK Kim BJ Jung HJ Kim JH Lee JS Lee WK Lee CH 《Chemical & pharmaceutical bulletin》2002,50(3):415-418
A new series of 1beta-methylcarbapenems 1a-d bearing 5-methyl-4-mercaptopyrrolidinone rings has been prepared and evaluated for in vitro antibacterial activity and pharmacokinetic parameters. Most compounds showed excellent antibacterial activity and high stability to dehydropeptidase-1. We have synthesized optically active 5-methyl-4-hydroxypyrrolidinones from enantiomerically pure aziridine esters. 相似文献
122.
The vanadium trapping effect of Mg and La containing additives in cracking catalyst contaminated with 2300 ppm Ni and 4700 ppm V has been analyzed by microactivity test (MAT) and imaging secondary ion mass spectrometry (SIMS). The results of SIMS imaging are consistent with cracking activity data and show that the La/spinel is a superior vanadium trap for the fluid catalytic cracking of hydrocarbons (FCC) operation. La/spinel serves as a dual function additive for both vanadium trap and SOx removal. The optimum amount of La/spinel added to the cracking catalyst is about 15% by weight. This results in an increased catalytic activity, an increase in gasoline yield, and a decrease in coke and gas factors. The MgAl2O4 phase of Mg/Al2O3 additive is found capable of trapping vanadium while its MgO of Mg/Al2O3 phase can migrate to zeolite particles (the active component of the FCC catalyst) that, in turn, causes a decline in the activity of that catalyst. 相似文献
123.
An efficient eight-step synthesis (53% overall) and the evaluation of 1,2,9,9a-tetrahydrocyclopropa[c]benz[e]-3-azaindol-4-one (CBA) and its derivatives containing an aza variant of the CC-1065/duocarmycin alkylation subunit are detailed. This unique deep-seated aza modification provided an unprecedented 2-aza-4,4-spirocyclopropacyclohexadienone that was characterized chemically and structurally (X-ray). CBA proved structurally identical with CBI, the carbon analogue, including the stereoelectronic alignment of the key cyclopropane, its bond lengths, and the bond length of the diagnostic C3a-N2 bond, reflecting the extent of vinylogous amide (amidine) conjugation. Despite these structural similarities, CBA and its derivatives were found to be much more reactive toward solvolysis and hydrolysis, much less effective DNA alkylating agents (1000-fold), and biologically much less potent (100- to 1000-fold) than the corresponding CBI derivatives. 相似文献
124.
Huang HS Chiou JF Chiu HF Hwang JM Lin PY Tao CW Yeh PF Jeng WR 《Chemical & pharmaceutical bulletin》2002,50(11):1491-1494
The synthesis of a series of anthraquinone moieties bearing symmetrical sulfur-linked substituents in the 1 and 5 positions is described. These compounds were evaluated for their ability to inhibit the growth of suspended rat glioma C6 cells and human hepatoma G2 cells, respectively. In addition, the redox property of the compounds was determined based on the inhibition of lipid peroxidation in model membranes. Compounds 2a and 2h in this series compared favorably and exhibited the most potent cytotoxicity (0.02, 0.05 microM) against C6 cells in the XTT colorimetric assay. As far as redox properties are concerned, all bis-thio-anthraquinones show potential lipid peroxidation in model membranes very close to that of mitoxantrone (MX), and 2a, 2d, 2e, 2i, 2j, and 2k have more potential than that of MX. The lack of cytotoxicity of compound 2i cannot be related to lipid peroxidation, but the steric and electronic properties of the side-chain substituent maybe impair effective recognition of the cleavable complex. In contrast to MX, 2a and 2h are cytotoxic in rat glioma C6 cells and do not enhance lipid peroxidation in model membranes. 相似文献
125.
Micropatterning of a semicrystalline poly(vinylidene fluoride) (PVDF) solution was performed by a temperature controlled capillary micromolding where the rate of solvent evaporation was controlled by substrate temperature. In order to choose proper solvents for micropatterning, we have investigated the solubility of PVDF in various organic solvents and crystal structures of the PVDF bulk films cast from the solvents. The films prepared from the polar solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO) dominantly showed γ type crystals regardless of preparation temperature, while the films from tetrahydrofuran (THF) exhibit α type crystals and the ones from acetone and methyl ethyl ketone (MEK) show the characteristics of both α- and γ-PVDF. The quality of micropatterns and shapes of the PVDF crystals in the patterns significantly depend on solvent evaporation rates. Micropatterns of PVDF formed in DMF at 120 °C showed the best uniformity in shape. Crystals of the PVDF nucleated at the center regions of microchannels tended to be elongated with the b-axis of γ-PVDF crystals along the channels as the concentration of the solution decreased. In contrast, crystals nucleated at the corner regions of the channels had their b-axis oriented perpendicular to the channels. In line patterns with the width of 2 μm, the corner nucleated crystals were dominant and a resulting bamboo-like crystalline microstructure was observed in which the b-axis of γ-PVDF crystals, fast growth direction, is oriented normal to the microchannels. The crystal structures of the bulk films and the micropatterns were characterized by X-ray diffractometer, Fourier transform infrared spectroscope in Attenuated Total Reflection mode, Polarized Optical and Scanning Electron Microscope. 相似文献
126.
Jung-Hoon Oh Mun-Chual Rho Hyeong Kyu Lee Jae Nyoung Kim Sang-Hun Jung 《Tetrahedron letters》2004,45(38):7077-7079
Asymmetric synthesis of four possible stereoisomers of (9,10)-epoxyheptadecan-4,6-diyn-3-one was accomplished, and the absolute configuration of the naturally occurring (9R,10S)-epoxyheptadecan-4,6-diyn-3-one (1) was elucidated. 相似文献
127.
Much improved catalytic carbon-carbon bond-forming reactions between aryl chlorides and Grignard reagents has been achieved using nickel(II) ion on nanoporous carbon. 相似文献
128.
Azide-functionalized PAMAM dendrons containing an azidopropylamine focal point were synthesized by the divergent method and applied for the construction of symmetric PAMAM-like dendrimers containing 1,2,3-triazole rings as connectors via stitching with two different multi-terminal alkynes. The stitching method was based on the click chemistry protocol, i.e., the copper-catalyzed cycloaddition reaction between an alkyne and an azide. 相似文献
129.
Direct reaction of iron pentacarbonyl with 1-N,N-disubstituted-cyclohexa-2,5-dicnecarboxamide and 1-N,N-disubstituted-cyclohexa-1,3-dienecarboxamide mixture (in which the disubstkuted group is diethyl or diphenyl) gave the isomeric tricarbonyl iron complexes of 2-N,N-disubstituted-1,4-η-cyclohexa-1,3-dienccarboxamide (1), 1-N,N-disubslituted-1,4-η-cyclohexa-1,3-dienecarboxamide (2), and 5-N,N-disubstituted-1,4-η-cyclohexa-1,3-dienecarboxamide (3) and tricarbonyliron complexes of l-N,N-disubstituted-cyclohexenecarboxamide (4). These complexes were separated and characterized by IR, UV-VIS, 1H NMR, elemental analysis, and mass spectra. Only 1 isomerized to give 2 under acidic conditions; both 1 and 2 undergo hydride abstraction with triphenylmethyl hexafluorophosphatc. Complexes 3, undergo neither isomcrization nor hydride abstraction. According to the spectral data, the possible interaction between carboxamide and iron carbonyl moiety was investigated. The irreversible electrochemical behavior of these complexes were studied. 相似文献
130.
Ronald A. Buono Nathalie Kucharczyk Magrit Neuenschwander Johan Kemmink Lih-Yueh Hwang Jean-Luc Fauchère Carol A. Venanzi 《Journal of computer-aided molecular design》1996,10(3):213-232
Summary The design of enzyme mimics with therapeutic and industrial applications has interested both experimental and computational chemists for several decades. Recent advances in the computational methodology of restrained molecular dynamics, used in conjunction with data obtained from two-dimensional 1H NMR spectroscopy, make it a promising method to study peptide and protein structure and function. Several issues, however, need to be addressed in order to assess the validity of this method for its explanatory and predictive value. Among the issues addressed in this study are: the accuracy and generizability of the GROMOS peptide molecular mechanics force field; the effect of inclusion of solvent on the simulations; and the effect of different types of restraining algorithms on the computational results. The decapeptide Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly, which corresponds to the sequence of ACTH1–10, has been synthesized, cyclized, and studied by two-dimensional 1H NMR spectroscopy. Restrained molecular dynamics (RMD) and time-averaged restrained molecular dynamics (TARMD) simulations were carried out on four different distance-geometry starting structures in order to determine and contrast the behavior of cyclic ACTH1–10 in vacuum and in solution. For the RMD simulations, the structures did not fit the NOE data well, even at high values of the restraining potential. The TARMD simulation method, however, was able to give structures that fit the NOE data at high values of the restraining potential. In both cases, inclusion of explicit solvent molecules in the simulation had little effect on the quality of the fit, although it was found to dampen the motion of the cyclic peptide. For both simulation techniques, the number and size of the NOE violations increased as the restraining potential approached zero. This is due, presumably, to inadequacies in the force field. Additional TARMD vacuum-phase simulations, run with a larger memory length or with a larger sampling size (16 additional distance-geometry structures), yielded no significantly different results. The computed data were then analyzed to help explain the sparse NOE data and poor chymotryptic activity of the cyclic peptide. Cyclic ACTH1–10, which contains the functional moieties of the catalytic triad of chymotrypsin, was evaluated as a potential mimic of chymotrypsin by measurement of the rate of hydrolysis of esters of L-and d-phenylalanine. The poor rate of hydrolysis is attributed to the flexibility of the decapeptide, the motion of the side chains, which result in the absence of long-range NOEs, the small size of the macrocycle relative to that of the substrate, and the inappropriate orientation of the Gly, His, and Ser residues. The results demonstrate the utility of this method in computer-aided molecular design of cyclic peptides and suggest structural modifications for future work based on a larger and more rigid peptide framework. 相似文献